Zeolites are crystalline aluminosilicate compositions which are microporous and which are formed from corner sharing AlO2 and SiO2 tetrahedra. Numerous zeolites, both naturally occurring and synthetically prepared, are used in various industrial processes. Synthetic zeolites are prepared via hydrothermal synthesis employing suitable sources of Si, Al and structure directing agents such as alkali metals, alkaline earth metals, amines, or organoammonium cations. The structure directing agents reside in the pores of the zeolite and are largely responsible for the particular structure that is ultimately formed. These species balance the framework charge associated with aluminum and can also serve as space fillers. Zeolites are characterized by having pore openings of uniform dimensions, having a significant ion exchange capacity, and being capable of reversibly desorbing an adsorbed phase which is dispersed throughout the internal voids of the crystal without significantly displacing any atoms which make up the permanent zeolite crystal structure. Zeolites can be used as catalysts for hydrocarbon conversion reactions, which can take place on outside surfaces as well as on internal surfaces within the pore.
A particular zeolite, IM-5, was first disclosed by Benazzi, et al. in 1996 (FR96/12873; WO98/17581) who describe the synthesis of IM-5 from the flexible dicationic structure directing agent, 1,5-bis(N-methylpyrrolidinium)pentane dibromide or 1,6-bis(N-methylpyrrolidinium)hexane dibromide in the presence of sodium. After the structure of IM-5 was solved by Baerlocher et al. (Science, 2007, 315, 113-6), the International Zeolite Structure Commission gave the code of IMF to this zeolite structure type, see Atlas of Zeolite Framework Types. The IMF structure type was found to contain three mutually orthogonal sets of channels in which each channel is defined by a 10-membered ring of tetrahedrally coordinated atoms, however, connectivity in the third dimension is interrupted every 2.5 nm, therefore diffusion is somewhat limited. In addition, multiple different sizes of 10-membered ring channels exist in the structure.
Applicants have successfully prepared a new family of materials designated UZM-44. The topology of the materials is similar to that observed for IM-5. The materials are prepared via the use of a mixture of simple commercially available structure directing agents, such as 1,5-dibromopentane and 1-methylpyrrolidine. UZM-44 may be used as a catalyst in processes for the conversion of low carbon number compounds, such as methane, to at least one aromatic compound, such as benzene.
Literature has proposed to produce aromatic compounds such as benzene, toluene and xylenes from petroleum naphtha streams. Attempts have also been made to produce useful aromatic compounds from low molecular weight aliphatic compounds by, for example, the pyrolysis of natural gas, acetylene and other gases. However, this technique produces benzene and other useful aromatic compounds in very low yields while producing large amounts of tar, insoluble carbon residue and high molecular weight aromatic compounds, all of which are of little commercial use. Specifically, in the pyrolysis of methane and acetylene, the reaction is carried out at a temperature of about 1,000° C. or higher with a conversion rate of only a few percent and a selectivity to naphthalenes of less than 1%, and thus has little practical application.
There are reports in the art of processes for converting natural gas into aromatic compounds. For example, U.S. Pat. No. 5,288,935 discloses a process for producing liquid hydrocarbons from natural gas, in which natural gas is first separated into a methane rich fraction and a C2+ fraction, the methane is then selectively oxidized with oxygen, the effluent from the selective oxidation is then mixed with a part of the C2+ fraction and the resulting mixture pyrolyzed to obtain an aromatic product. The final step is carried out at a temperature of about 300° C. to about 750° C. in the presence of an aromatizing catalyst consisting essentially of a zeolite, gallium, at least one metal from the Group VIII metals and rhenium and at least one additional metal selected from the group consisting of: tin, germanium, lead, indium, thallium, copper, gold, nickel, iron, chromium, molybdenum and tungsten; an alkaline metal or alkaline earth metal and an aluminum matrix.
It is also known that the non-oxidative conversion of methane to benzene via dehydroaromatization can be carried out using Mo/HZSM-5, see L. Wang, L. Tao, M. Xie, G. Xu, J. Huang, and Y. Yu Catal. Lett. 1993, 21, 35 and that dehydrocondensation of methane, optionally in the presence of CO or CO2, to form benzene and naphthalene can be carried out using a molybdenum/HZSM-5 or iron/cobalt modified Mo/HZSM-5, see S. Liu, Q. Dong, R. Ohonishi and M. Ichikawa, Chem. Commun. (1998), p. 1217-1218, and S. Liu, L. Wang, Q. Dong, R. Ohonishi, and M. Ichikawa, Stud. Surf Sci. Catal., Vol. 119, p. 241-246. These catalysts are known to deactivate both by coking and by damage from the repetitive regenerations required in the process. In contrast to this art, a catalyst which comprises a UZM-44 zeolite and which optionally can contain a promoter such as iron, cobalt, tungsten, or molybdenum can be used to successfully catalyze the conversion of at least one low carbon number aliphatic hydrocarbon to at least one aromatic compound. In addition less deactivation under process conditions may be observed than typical with MFI based catalysts.